Why was "You will eat the 'grass' of the field" a curse, considering that earlier (in Bereishis 1:29), Hashem told him that He had blessed him by giving him the 'grass' of the field?

1.

Rashi, Moshav Zekenim : The Pasuk here is merely elaborating on the curse mentioned in the previous Pasuk - and what it means is that when Adam would plant legumes or vegetables, it would produce thorns and thistles and other kinds of grasses, which he would be forced to eat.

2.

Targum Yonasan: Initially, Adam was cursed in that he would have to munch grass directly from the ground together with his animals. However, after Davenning to Hashem to distinguish between him and them, his prayers were accepted. Instead, he would have to work by the sweat of his brow in order to prepare his food.
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